The invention disclosed herein relates to the field of nuclear magnetic resonance (NMR) well logging apparatus and methods. More specifically, the invention is related to acquiring and processing NMR signals to estimate a particular property of an earth formation penetrated by a borehole.
Exploration and production of hydrocarbons generally requires precise and accurate measurements of earth formations, which may contain reservoirs of the hydrocarbons. Boreholes are typically drilled into the earth formations for the exploration and production.
Well logging is a technique used to perform measurements of an earth formation penetrated by a borehole. In one embodiment, referred to as logging-while-drilling (LWD), a logging tool is used to perform the measurements during drilling operations. The logging tool is attached to a drill string having a drill bit. Thus, as the drill string rotates to turn the drill bit, the logging tool also rotates with the drill string. Because of the rotation, most LWD measurements may then be averaged about the 360 degrees of rotation to produce a value of a property of the earth formation. Especially most LWD nuclear magnetic resonance (NMR) logging tools use axisymmetric magnetic fields. Despite the tool rotating with the drill string, the magnetic fields do not vary in the region of investigation and, hence, the rotation does not influence the NMR measurement. Thus, the tools yield an average of the formation property.
Many types of measurements may be performed with the well logging technique. One type of measurement measures the response of nuclei in the earth formation to a changing magnetic field in a technique known as nuclear magnetic resonance (NMR) measurements. NMR measurements in LWD applications are usually implicitly averaged about the 360 degrees of rotation to produce a measured NMR property assumed to be symmetric around the borehole at a certain depth. However, this assumption may not be accurate. Properties of the earth formation may not be symmetric around the borehole.
Therefore, what are needed are techniques that measure NMR properties of an earth formation where the NMR properties are asymmetric about the borehole. Preferably, the techniques associate an azimuthal direction with each NMR measurement.